Our research focuses on the role of the slow response in causing cardiac arrhythmias. The slow response is a slowly propagating action potential that occurs in cardiac fibers that have an abnormally low resting potential. Having spent time in studying the slow response in fibers exposed to sodium-free solutions we are once again studying it in fibers exposed to normal ionic environments. We are particularly interested in what we have called triggered arrhythmias in which each action potential grows out of an afterpotential created by the previous action potential. We continue to search for areas in the heart where this sort of behavior can be found since it appears that it may be a very important cause of abnormal rhythms of the heart. We are also interested in abrupt and discontinuous changes in resting potential since marked loss of resting potential sets the stage for the slow response to appear. The sensitivity of triggered arrhythmias to antiarrhythmic drugs is also under active investigation. BIBLIOGRAPHIC REFERENCES: Wit, A.L., Wiggins, J.R. and Cranefield, P.F. Some effects of electrical stimulation on impulse initiation in cardiac fibers; its relevance for the determination of the mechanisms of cardiac arrhythmias. In: Wellens, H.J.J., Lie, K.I. and Janse, M.J. (eds.) The conduction system of the heart. Philadelphia, Lea and Febiger, 1976, pp. 163-181. Wiggins, J.R. and Cranefield, P.F. Two levels of resting potential in canine cardiac Purkinje fibers exposed to sodium-free solutions. Circulation Res., 1976, in press.